1. Field of the Invention
This invention in general relates to a process for recovering cobalt catalysts, and more specifically to an improved process for recovering and recycling cobalt oxo catalysts.
2. DESCRIPTION OF THE PRIOR ART
In the well known oxo process, olefins are hydroformylated by reaction with carbon monoxide and hydrogen, generally charged as syn gas mixtures, in the presence of a cobalt oxo catalyst in dissolved form to form a mixture of oxo aldehydes and alcohols. This oxo reaction is typically carried out at syn gas pressures of from about 1500 to 4500 psig and at temperatures of from about 65.degree. to 230.degree. C. Thereafter, the product mixture containing the alcohols and aldehydes is recovered and can then be treated by known means to hydrogenate the aldehydes to form additional quantities of the corresponding alcohols. These alcohols, in turn, are widely used as chemical intermediates in the manufacture of plasticizers, detergents, solvents and the like.
Prior to the hydrogenation step, the crude oxo reaction effluent, which contains dissolved cobalt catalysts, the product aldehyde and alcohol and reaction by-products together with any metallic contaminants, is generally treated to remove the dissolved cobalt catalyst, which then for reasons of economy must be recycled to the oxo reactor.
A wide variety of catalyst recovery/recycle processes have therefore been developed. U.S. Pat. No. 2,751,403 is directed to a process in which cobalt is removed from crude oxo products by extraction with an aqueous acid such as acetic acid to form an aqueous extract containing cobalt in both the cationic and anionic forms, viz., as the anion [Co(CO).sub.4 ].sup.- and the corresponding cobalt salt, cobaltous bis-tetracarbonylcobaltate, Co.sup.++ [Co(CO).sub.4.sup.- ].sub.2. The aqueous extract is then subjected to oxidation with air or O.sub.2 at 38.degree. to 66.degree. C. and at a pH of 5-6 with the addition of a higher molecular weight carboxylic acid salt, e.g., sodium oleate, to convert anionic cobalt to the Co.sup.++ form and to achieve substantially quantitative recovery of the cobalt as a cobalt soap, e.g., cobaltous oleate, which was the desired catalytic species. The cobalt soap is then extracted into an organic liquid for recycle to the oxo reactor.
However, there has been continuous interest in other, lower cost alternatives to such expensive cobalt soaps, which during use are converted to other forms and have to be reconverted to the soap during the recycle process.
Another form of cobalt useful as oxo catalyst is dicobalt octacarbonyl, Co.sub.2 (CO).sub.8.
U.S. Pat. No. 3,265,468 discloses a process for producing Co.sub.2 (CO).sub.8 from an aqueous solution of an alkali metal carbonyl of cobalt (e.g., NaCo(CO).sub.4) by acidifying the solution with a mineral acid, followed by extraction, e.g., with toluene, to form an organic phase containing Co.sub.2 (CO).sub.8.
In U.S. Pat. No. 3,793,437, crude oxo effluent containing cobalt is contacted with an aqueous solution of metallic extracting agents, such as various metal salts and certain zeolites, in the presence of H.sub.2 and CO to form an aqueous salt of carbonyl cobaltate which is subsequently decomposed with an organic acid or a mineral acid to a water-soluble cobalt hydridocarbonyl. The aqueous solution containing the cobalt hydridocarbonyl is then heated in the present of CO and a water-immiscible organic solvent to form dicobalt octacarbonyl which is extracted into the organic solvent. After further treatment (e.g., drying, dilution or concentration), the organic solvent can be recycled to the oxo reactor. The patentees indicate that the thus-recovered cobalt carbonyl is sensitive to oxygen or oxidizing agents and must be protected from such oxidants as by degassing or by replacement of the ambient atmosphere with inert gas.
R. Kummer, et al., "New Hydroformylation Technology with Cobalt Carbonyls," Homogeneous Catalysis--II, Advances in Chemistry Series No. 132 (D. Forster et al.), pp. 19-26 (A.C.S. 1973) relates to a BASF process in which crude oxo product is demetalled at 120.degree. C. and 10 atm. with air and an aqueous formic acid/cobaltous formate solution, and the resulting aqueous Co.sup.++ formate solution is reacted with CO and H.sub.2 to preform anionic cobalt, Co(CO).sub.4.sup.-, in the solution, which is then subjected to an olefin extraction to give an olefin phase containing cobalt as either Co.sub.2 (CO).sub.8 or (at low CO pressure) Co.sub.4 (CO).sub.12. Kummer et al. describes the crude oxo product demetalling step by the following equation (I): EQU Co.sub.2 (CO).sub.8 +O.sub.2 +4H.sub.3 O.sup.+ +4HCOO.sup.- .fwdarw.2Co.sup.++ +4HCOO.sup.- +6H.sub.2 O+8CO (I)
The authors point out that the resulting aqueous phase contains all the cobalt and that only water-soluble Co.sup.++ compounds are formed.
German Pat. No. 1,272,911 to BASF, as cited at 69 Chem. Abs. 95964d (1968), describes the demetalling of a crude oxo product at 116.degree. C. and 30 atm. with air, acetic acid, water and a recycled Co.sup.++ salt solution, using a residence time in the demetalling zone of 3 seconds, to give an organic phase containing practically no cobalt.
The following BASF patents relate to similar processes: U.S. Pat. No. 3,941,848; British Pat. Nos. 1,383,658 and 1,390,898; and German Offenlegungschrift No. 2,451,473 (1976).
U.S. Pat. No. 4,255,279 contacts a crude oxo effluent in a first step with an aqueous Co.sup.++ salt of an organic or inorganic acid to extract cobalt into the aqueous phase. After separation from the thus-treated crude oxo product, the aqueous phase, which contains cationic and anionic cobalt, Co.sup.++ and Co(CO).sub.4.sup.-, is treated with syn gas to preform additional Co.sup.++ into the anionic, Co(CO).sub.4.sup.-, form. The preformed effluent is then contacted with an organic solvent to extract cobalt carbonyls therefrom into the organic phase for ultimate recycle to the oxo reactor. The treated crude oxo product obtained from the first step still contains some cobalt in an oil-soluble form, e.g., dicobalt octacarbonyl, and is further demetalled by treatment at 65.degree. to 93.degree. C. with an aqueous organic or inorganic acid and oxygen to oxidize the cobalt to a water-soluble form, e.g., Co.sup.++ salt of the selected acid. The patentees indicate that substantially all of the cobalt is thereby separated from the organic layer, resulting in an oxo product containing cobalt in a concentration of about 10 ppm or less.
Japanese Patent Publication 73/17,594 (May 30, 1973) oxidizes a cobalt hydrocarbonyl water-soluble metal salt (e.g., NaCo(CO).sub.4 or Co[Co(CO).sub.4 ].sub.2) in aqueous solution with air or O.sub.2 to form dicobalt octacarbonyl solids, followed by extraction thereof using an organic solvent or raw material olefin or their mixture. Alternatively, the patentees indicate that the organic solvent can be added in the oxidation step to extract the dicobalt octacarbonyl directly into the organic solvent layer.
In R. B. King, Organometallic Synthesis, vol. 1, p. 98 (Academic Press 1965), it is indicated that Co.sub.2 (CO).sub.8 crystals are soluble in organic solvents; are unstable to both thermal decomposition and air oxidation; and rapidly lose CO at 50.degree. C. to form Co.sub.4 (CO).sub.12 and ultimately cobalt metal. On exposure to air for several minutes, Co.sub.2 (CO).sub.8 crystals are said to be oxidized to a Co.sup.++ derivative, which is presumed to be either the oxide or the carbonate. Also, Co.sub.2 (CO).sub.8 crystals, when isolated by crystallization from organic solvents, are said to be pyrophoric if obtained as finely divided crystals. A preparatory procedure is therefore suggested in which cobalt (II) acetate tetrahydrate is reacted at 160.degree.-180.degree. C. with CO and H.sub.2 to form acetic acid and Co.sub.2 (CO).sub.8 crystals, which are isolated by filtration under N.sub.2.
W. Hieber and W. Hubel, Zeitschr. Elektrochem. 57, no. 4; pp. 235-243 (1953) indicate that solutions of cobalt carbonyl hydride are very sensitive to oxidizing agents and that dimeric cobalt carbonyl flakes are immediately formed from even minute traces of atmospheric oxygen (see Section I, paragraph 2).